28.
The answer is: C
Explanation
The correct option is C: Valid.
According to Section 25(3) of the Indian Contract Act, 1872, a promise made in writing and signed by the person to be charged therewith, or by his agent generally or specially authorized in that behalf, to pay wholly or in part a debt of which the creditor might have enforced payment but for the law for the limitation of suits, is a valid contract.
Therefore, X's written promise to pay Y Rs. 15,000 on account of the debt barred by the Limitation Act is a valid contract and can be enforced by Y. However, Y can only claim Rs. 15,000 and not the original amount of Rs. 20,000 as per the new contract.